A crater is a circular depression found at the summit or on the flanks of a volcano. It serves as the primary exit point for volcanic materials. Most craters are formed by the explosive force of gas and magma blasting away the top of the volcanic cone. They can range in size from a few meters to over a kilometer in diameter. When the underlying magma chamber empties and the summit collapses, the crater can transform into a much larger basin called a caldera. Craters are the most recognizable feature of volcanic peaks and often contain smaller secondary vents.